Security documents often incorporate an authentication device that includes an optically variable feature such as diffraction gratings or holographic optical microstructures. These features can exhibit optically variable effects such as colour changes, movement effects, and distinct switches between images.
Certain optically variable features may include polymer or laminate microstructures in the form of foils that exhibit colour shifts in transmitted light passing through the foil and/or ambient light reflecting from the foil. Tilting the foil results in a visible colour-shift effect due for example to a laminate microstructure, or Bragg stacking within the foil. Such features provide useful surface security features in applications where the substrate to which they are applied is flexible or foldable, such as in banknotes.
Further known authentication devices include optical elements that include various circuits or circuit components, wherein application of a current to the circuit or circuit components causes a change in the properties of the optical elements. Such devices include an internal or external associated power source to supply electric current to the circuit and its components to cause the change in optical properties. These electrical authentication devices typically directly electrically connect a piezoelectric material (as the power source) with a state changing material (i.e., an active layer) such an electrophoretic, polymer dispersed liquid crystal. However, these traditional solutions use electrodes and electrical circuits to conduct the charge generated by the piezoelectric material to the state changing material.
Although these traditional solutions produce a change in the state (colour change, character appearance, etc.) of the active layer, such approaches suffer a significant design flaw since the authentication device's operation is vulnerable to failure should one of the electrodes, which conduct electrical charge to the active layer, be cut or broken. Further, these traditional authentication devices have not been capable of surviving the commonly known “crumpling tests” performed as part of durability testing for banknotes.
There is a continuing need to improve authentication devices for security documents that are difficult to counterfeit, yet relatively inexpensive to manufacture, and are suitable for application to a range of substrate materials including both paper and polymer films.